Method and system for using computed tomography to test pulmonary function

ABSTRACT

Computed axial tomography images of different respiratory phases of lungs are obtained, where the intensity of the image measures lung density. One image is deformed to the coordinate space of the other image, and the differences between the intensity values of the other image as compared to the mapped image are evaluated as measures of ventilation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to methods and systems of determining pulmonary function, and in particular to methods and systems using computed tomography.

2. Background Description

Currently, pulmonary ventilation is performed using single photon emission computed tomography (SPECT), a nuclear medicine imaging procedure, which is more expensive and time consuming than computed axial tomography (CAT scan, or CT). Furthermore, the resolution of SPECT is significantly poorer than that of CT. What is needed is a method for measuring lung ventilation that uses CT.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to measure pulmonary function using computed axial tomography (CT).

This invention is brings together two emerging technologies, thoracic four dimensional computed tomography (4DCT) and deformable-image registration algorithms, to devise a new clinical test for lung function. The lung function tests will show the difference in lung density between respiratory phases in different parts of the lung, from which regions of small or no density change (low ventilation) can be identified.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which:

FIG. 1 a is an exhale image;

FIG. 1 b is an inhale image;

FIG. 1 c is the exhale image deformed to the inhale image;

FIG. 1 d is a difference image between the inhale image and the deformed exhale image;

FIG. 1 e is a smoothed difference image.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Four-dimensional computed tomography (4DCT) acquisition methods that explicitly account for respiratory motion have been recently developed in academic and commercial settings. Similarly, deformable image registration algorithms are evolving to the point of routine clinical utility. The combination of these two emerging technologies can be used as a pulmonary function test for ventilation by assessing the density differences of the same anatomic areas of the lung from CT scans acquired at different respiratory phases.

Due to the deformation of the lungs caused by respiration, the CT scans at different respiratory phases (referred to subsequently as inhale and exhale scans) cannot be directly compared as the anatomy is in a different location in the two images. FIG. 1 a and 1 b, respectively, show exhale 110 and inhale 120 images. However, deformable image registration algorithms can be applied to deform one respiratory phase to another. This is shown by the image 130 in FIG. 1 c, where the exhale image 110 of FIG. 1 a is deformed into the inhale image 120 of FIG. 1 b. This enables calculation of the density differences between the inhale image 120 (shown in FIG. 1 b) and the deformed exhale image 130 (shown in FIG. 1 c) based on the differences of the intensity values in the images, and hence ventilation can be quantified.

For example, let the exhale CT scan be given by I(x_(exhale)), and the inhale scan by I(x_(inhale)) . A transformation, u, can be found that maps the coordinate space of I(x_(exhale)) to I(x_(inhale)) via u(x_(exhale)→x_(inhale)). The ventilation can then be quantified by evaluating the difference between the two images I(x_(inhale))−I(u(x_(exhale))) in the lung region.

Note that the vector displacement fields themselves, and variations thereof, could also be used to analyze lung function. Furthermore, for this method it is prudent to use a deformable image registration algorithm which is not driven by minimizing intensity differences, as such algorithms will artificially try to minimize the intensity difference, which for this application is the quantity of interest.

A proof-of-principle example of this method is given in FIG. 1 in which the exhale image 110 (FIG. 1 a) has been deformed to the inhale image 120 (Fig. 1 b) creating the image 130 shown in FIG. 1 c. A subtraction of the deformed exhale image 130 (FIG. 1 c) from the inhale image 120 (FIG. 1 b) results in a difference image 140 (FIG. 1 d). A smoothing function is then applied to minimize the effects of CT artifacts and limitations in the image registration process, resulting in a smoothed difference image 150 (FIG. 1 e).

While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims. 

1. A method for quantifying pulmonary ventilation, comprising: obtaining computed axial tomography (CT) images of first and second respiratory phases; deforming the first phase image to the second phase image; and quantifying the differences between the deformed image and the second phase image.
 2. The method of claim 1, wherein the differences quantified are differences in lung density as measured by image intensity.
 3. The method of claim 2, wherein a deformable image registration algorithm is used for said deforming.
 4. The method of claim 3, wherein the deformable image registration algorithm is not driven by minimizing differences in image intensity.
 5. The method of claim 1, further comprising smoothing the differences between the deformed image and the second phase image before quantifying the differences.
 6. The method of claim 1, wherein said first phase is an exhale phase and said second phase is an inhale phase.
 7. The method of claim 1, wherein the deforming is accomplished using a transformation that maps a coordinate space of the first image to a coordinate space of the second image.
 8. The method of claim 7, wherein the quantifying is accomplished by evaluating the differences between image intensity values in the coordinate space of the second image of the second image as compared to the mapped first image.
 9. A system for quantifying pulmonary ventilation, comprising: means for obtaining computed axial tomography (CT) images of first and second respiratory phases; means for deforming the first phase image to the second phase image; and means for quantifying the differences between the deformed image and the second phase image.
 10. The system of claim 9, wherein the differences quantified are differences in lung density as measured by image intensity.
 11. The system of claim 10, wherein a deformable image registration algorithm is used by said deforming means.
 12. The system of claim 11, wherein the deformable image registration algorithm is not driven by minimizing differences in image intensity.
 13. The system of claim 9, further comprising means for smoothing the differences between the deformed image and the second phase image before quantifying the differences.
 14. The system of claim 9, wherein said first phase is an exhale phase and said second phase is an inhale phase.
 15. The system of claim 9, wherein the deforming means uses a transformation that maps a coordinate space of the first image to a coordinate space of the second image.
 16. The system of claim 15, wherein the quantifying means evaluates the differences between image intensity values in the coordinate space of the second image of the second image as compared to the mapped first image. 